The present invention relates to a vapor phase soldering machine, and more particularly to a vapor phase soldering machine that utilizes a tertiary cooling vapor to provide for rapid cool-down and hardening of the molten soldered connections of surface mounted devices to a printed circuit board, thereby providing for higher integrity connections.
In connecting various types of electronic components, both leaded and leadless, to a printed circuit board, various methods have been utilized to obtain good quality soldered joints. The most basic method is to use a hand-held soldering iron and a spool of solder wire. The solder wire generally contains a compound mixed within the solder known as "flux". The flux performs a cleaning action as the solder joint is being made. The person making the soldered connection touches all elements of the joint simultaneously with the tip of the soldering iron. The tip is normally held at a temperature of approximately 600.degree.-800.degree. F. Once the joint is heated to above the melting point of the solder wire being used (typically 500.degree.-575.degree. F.), the person feeds the solder wire into the joint, thereby melting it and causing it to flow around the joint elements, such as the lead from the component and the corresponding plated through hole in the circuit board. Once the correct amount of solder has been applied, the person removes the soldering iron tip from the joint and the joint cools via convective, conductive and radiant cooling.
This cooling action causes the molten solder to solidify within 3-5 seconds. This cooling rate is rapid enough to effect a joint which causes the solder to properly fuse to all of the joint elements. A typical test for a properly soldered connection is a shiny, smooth and lustrous appearance with a slightly concave configuration, even when viewed under magnification. Perhaps the most telling characteristic of a good solder joint is a fine grain structure, together with the bright finish. On the other hand, a weak solder joint is characterized by a coarse grain structure having crystal-like formations. Typical military specifications for electronic equipment prepared by government contractors base their pass/fail inspection criteria of soldered connections upon this visual, subjective test. It is known that the quicker the cooling rate, the better the metallurgical grain structure of the resulting joint. Further, it is known that a cooling rate of 3-5 seconds forms such a small grained structure. It is this small grain structure that causes the shiny, smooth and lustrous appearance of the soldered joint.
However, hand soldering is slow and tedious at best, and can produce only one soldered connection at a time by the worker. Therefore, it is known in the electronics assembly industry to utilize more automated methods of simultaneously soldering a plurality of electrical connections. These automated soldering technologies are categorized by the package design of electrical components (i.e., joint elements) that are to be soldered.
The first type of package design of electrical components is the oldest category and is referred to as a "leaded component" design. That is, each component utilizes lead wires that project from the body of the component. For example, discrete elements such as resistors, capacitors and diodes have lead wires exiting the body either axially or radially. Also, integrated circuits have pins arranged around the perimeter of the integrated circuit package. With a leaded component, the lead is inserted into connective holes formed in a printed circuit board. These holes are drilled through the printed circuit board, which is usually made of an insulative material such as fiberglass. The plated through holes are plated with an electrically-conductive material, such as a combination of tin-lead, to allow continuity between the hole and various conductive traces on the printed circuit board surfaces that connect together the components on the board. When the component wire lead is soldered to the plated through hole, the resulting joint becomes an electrical conductor.
However, while hand soldering typically produces a sound metallurgical joint, the relatively large amount of time needed to solder all of the connections on a circuit board manually by hand precludes hand soldering from usage where a large number of assembled and soldered printed circuit boards must be produced, each with a multiple of connections.
Therefore, it is known to use an automated technology for soldering leaded components into plated through holes on printed circuit boards. A common technique is wave soldering, which is carried out using a machine having a constant speed conveyor. The printed circuit board with the components assembled in place, but not yet soldered, goes onto the conveyor in a specially designed holding fixture. The conveyor is started and the printed circuit board enters the wave soldering process.
Initially, the printed circuit board rides over a turbulating wave of liquid flux. The flux is a compound that cleans all of the elements of a solder joint. In the next step, the printed circuit board enters a preheat area of the wave soldering machine where heat lamps raise the temperature of the printed circuit board. This preheat area conditions the printed circuit board to approximately 300.degree. F., to prevent damaging thermal shock to the board and components when the board enters the next step of the process.
Next, the board travels over a turbulating wave of molten solder. All component wire leads projecting through the plated through holes in the printed circuit board are simultaneously soldered as the solder from the turbulating waves wicks up the leads and into the holes. Once the board travels over the turbulating solder wave, the solder joints cool rapidly, either through the process of natural convection, conduction and radiation, or through forced air convection cooling (e.g., similar to a fan).
A wave soldered connection typically cools as rapidly as a hand soldered connection; i.e., approximately in 3-5 seconds. Similar to a hand soldered joint, the joint produced by wave soldering is "wetted" and has a shiny, smooth and lustrous appearance due to the small metallurgical grain structure of the solder joint. Therefore, wave soldered joints generally meet all military specification visual inspection requirements.
A more modern type of component technology is known as surface mounted devices ("SMD"). These components are designed without wire leads protruding from the body of the component. Instead, these SMD components incorporate integral pads that are directly attached to the body of the component. These pads serve to mount the SMD to the printed circuit board, thus making all necessary connections.
Surface mounted printed circuit boards have corresponding mating pads disposed on one or more board surfaces that coincide with the component mounting pads. A solder paste is applied to the printed circuit board pads, either by screen printing, stenciling, or dispensing with a syringe. The solder paste is applied to the printed circuit board pads prior to the placement of the SMDs onto the board. Once the components are placed onto the board, the board is then soldered utilizing one of a number of techniques, among the most popular being vapor phase soldering.
The vapor phase soldering process begins once all of the SMDs have been placed on a printed circuit board that has the solder paste applied at the appropriate locations on the board. In a typical prior art vapor phase soldering machine, as exemplified in U.S. Pat. No. 3,904,102, the board is placed in a wire basket that descends initially downward into the machine. The printed circuit board contacts a preheat area that comprises a vapor, called a secondary vapor, that is approximately 300.degree. F. This exemplary temperature is usually less than that required to actually melt the solder paste.
After the temperature conditioning of the printed circuit board and the components thereon in the secondary vapor portion of the machine, the printed circuit board descends further downward into the machine and enters a vapor, called a primary vapor, with a temperature equal to or greater than the melting temperature of the solder paste. This primary vapor liquifies the solder paste and causes the component connection pads and the corresponding printed circuit board mounting pads to fuse together, thereby forming a plurality of discrete electrically continuous soldered connections. Once the solder paste has melted, the printed circuit board is moved back upward through the secondary vapor and out of the machine into the ambient atmospheric temperature. This causes the solder joints to cool, but rather slowly, as compared to both hand soldering and wave soldering. The typical solder joint cool-down time for the aforedescribed prior art vapor phase soldering machine is approximately 10-15 seconds.
This slow cool-down is inherent in the design of this prior art vapor phase soldering machines. When using hand soldering techniques, the heat required to reflow the solder is applied at a point on the board (i.e., the actual joint) using a pointed soldering iron tip. Only the area in the immediate vicinity of the joint being soldered heats up to the melting temperature. The rest of the circuit board remains at or near room temperature. Thus, the cooler circuit board surrounding the soldered joint acts as a heat sink to aid in the rapid cooling of the hand soldered joint. As mentioned hereinbefore, a hand soldered joint cools in approximately 3-5 seconds.
In contrast, vapor phase soldering machines, like those described in the aforementioned prior art '102 patent, are entirely immersed in the primary vapor that is hot enough to reflow solder. This means that the entire printed circuit board heats up to this temperature. Thus, to cool the individual joints that have just been soldered, the entire circuit board must be cooled. The '102 patent is problematic in that the printed circuit board must pass back through the secondary vapor before exiting the machine, where the board can cool. This dwell time in the secondary vapor contributes to the overall time in cooling the molten soldered joints. The prior art is devoid of teaching that provides for cooling of vapor phase soldered joints at a rate that is equivalent to that achieved by hand or wave soldering.
Due to the very slow cool-down cycle (i.e., 10-15 seconds) achievable by typical prior art vapor phase soldering machines, the resulting solder joints formed are metallurgically large grained, and have a low luster, non-shiny, grainy appearance. However, it should be noted that, even though these vapor phase solder joints may not meet military specification visual inspection criteria, they, nonetheless, may perhaps be of good quality, and be reliable soldered connections. But, because of the process-driven appearance of vapor phase soldered joints, it is difficult to correlate military specification pass/fail criteria to the relatively younger vapor phase soldering process based on military specification requirements for smooth, shiny and lustrous joints.
Accordingly, it is a primary object of the present invention to provide an improved vapor phase soldering machine that provides for a cool-down time of the soldered connections that is equivalent to that achievable using hand or wave soldering.
It is a general object of the present invention to provide an improved vapor phase soldering machine that provides for high quality solder joints as determined by a subjective visual inspection of the joints.
It is another object of the present invention to provide an improved vapor phase soldering machine that provides for high speed, economical soldering of printed circuit boards having a plurality of soldered connections.
It is yet another object of the present invention to provide an improved vapor phase soldering machine that provides high quality solder joints between surface mounted devices and the corresponding locations on a surface of a printed circuit board.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.